Spaceborne SAR anatomy of a city
Daniele Perissin(1) , Claudio Prati(1)
, and Alessandro Ferretti(2)
Politecnico di Milano,
Piazza L. da Vinci 32,
(2) Tele-Rilevamento Europa T.R.E., Via V. Colonna 7, 20149 Milano, Italy
The ensemble of stable radar targets (the so-called Permanent Scatterers -PS-) in urban area can be interpreted as the radar skeleton of a city. By means of the PS technique, recently developed at POLIMI, the skeleton radar reflectors are exploited for overcoming the difficulties related to the conventional SAR interferometry approach (phase decorrelation and atmospheric effects), achieving millimetre accuracy in monitoring possible relative displacements. Even if the PS technique is an operational tool since 2000, the physical nature of the targets is still a subject of investigation. A good knowledge of the PS physical nature is a key step for a correct interpretation of the measured deformation mechanism. As an example, the phase of a dihedral formed by the ground and a building wall does not change in presence of buildings slow subsidence, but it changes in case of ground subsidence. In second instance, the classification of reflecting structures behaving as PS allows an a-priori identification of the PS’s looking at the structural details of buildings. Finally, if we know the PS’s physical nature, we can foresee their electromagnetic behavior under different acquisition geometries, frequencies and polarizations, and we can develop feasibility studies on the integration of interferometric SAR multiple sensors (e.g. the future Radarsat 1 and Radarsat 2 coherent exploitation). In this work we present the last results of this research program obtained by using 300 ERS and ENVISAT repeated acquisitions on the area of Milano (Italy). First, the PS’s position and their elevation with respect to the ground are estimated with high precision. Then the scattering pattern of targets as a function of their geometrical characteristics (dimensions and orientation) is analysed. The variation of the Radar Cross Section as a function of the acquisition geometry (Normal Baseline and Doppler Centroid) is linked to the extension of the scatterer in range and azimuth. The targets radar response has been analysed by exploiting a frequency change (ERS and ENVISAT) and a different incidence angle (two different parallel tracks). The ERS-ENVISAT cross interferogram is used as an estimator of the sub cell range position of the scatterers. Exploiting the auto-interferogram generated from the ENVISAT Alternating Polarization acquisition, odd and even bounces of radar echoes are identified thus separating dihedrals from specular or trihedral reflectors [Souyris]; their different behaviour with temperature and height has been characterized. Finally, targets like poles visible from ascending and descending passes have been identified.